It is standard to package blood, saline solution, and even some liquid foodstuffs in a bag or pouch whose outlet is formed by a tube that is provided with a clamp. For use the end is cut off the tube and the clamp is then used to block or unblock flow from the tube. Since the container itself is disposable, the clamp must be made as cheaply as possible.
The standard solution is simply to stamp a keyhole-shaped hole in a flat strip of a rigid material. When the tube is positioned in the wide circular end of this hole flow through the tube is possible, and when it is pinched in the slot end of the hole such flow is blocked.
The main problem with such a clamp is that it represents a considerable hindrance. When in the blocking or tube-closed position it extends crosswise of the tube and makes handling and packaging the assembly fairly difficult. In fact it is not uncommon for this transversely projecting rigid item to poke into the pouch and pierce it, thereby destroying the product.